Conventional refrigeration technology typically utilizes a heat pump that relies on compression and expansion of a fluid refrigerant to receive and reject heat in a cyclic manner so as to effect a desired temperature change or i.e. transfer heat energy from one location to another. This cycle can be used to provide e.g., for the receiving of heat from a refrigeration compartment and the rejecting of such heat to the environment or a location that is external to the compartment. Other applications include air conditioning of residential or commercial structures. A variety of different fluid refrigerants have been developed that can be used with the heat pump in such systems.
While improvements have been made to such heat pump systems that rely on the compression of fluid refrigerant, at best such can still only operate at about 45 percent or less of the maximum theoretical Carnot cycle efficiency. Also, some fluid refrigerants have been discontinued due to environmental concerns. The range of ambient temperatures over which certain refrigerant-based systems can operate may be impractical for certain locations. Other challenges with heat pumps that use a fluid refrigerant exist as well.
Magneto caloric materials (MCMs)—i.e. materials that exhibit the magneto caloric effect—provide a potential alternative to fluid refrigerants for heat pump applications. In general, the magneto caloric effect refers to a process of entropic change whereby the magnetic moments of an MCM will become more ordered under an increasing, externally applied magnetic field and cause the MCM to generate heat. Conversely, decreasing the externally applied magnetic field will allow the magnetic moments of the MCM to become more disordered and allow the MCM to absorb heat. Some MCMs exhibit the opposite behavior—i.e. generating heat when the magnetic field is removed (which are sometimes referred to as para-magneto caloric material but both types are referred to collectively herein as magneto caloric material or MCM). The theoretical Carnot cycle efficiency of a refrigeration cycle based on an MCM can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. As such, a heat pump system that can effectively use an MCM would be useful.
Challenges exist to the practical and cost competitive use of an MCM, however. In addition to the development of suitable MCMs, equipment that can attractively utilize an MCM is still needed. It is desirable to provide for the transfer or heat to and from the MCM preferably in a continuous manner so that the equipment does not operate in a start and stop fashion that can be inefficient. Also, currently proposed equipment may require relatively large and expensive magnets, may be impractical for use in e.g., appliance refrigeration, and may not otherwise operate with enough efficiency to justify capital cost.
Additionally, as stated above, the ambient conditions under which a heat pump may be needed can vary substantially. For example, for a refrigerator appliance placed in a garage or located in a non-air conditioned space, ambient temperatures can range from below freezing to over 90° F. Some MCMs are capable of accepting and generating heat only within a much narrower temperature range than presented by such ambient conditions.
Also, the amount of MCM that needs to be magnetized can change as ambient conditions change. For example, for a refrigeration appliance, as the ambient temperature decreases, the amount of MCM that must be magnetized to properly maintain the temperature of food items stored inside the appliance can also decrease. As energy is required to magnetize the MCM, efficiency is reduced when more MCM than necessary is magnetized. However, no practical solutions for controlling the amount of MCM material to be magnetized has been proposed—particularly for a continuously operating machine.
Accordingly, a heat pump system that can address certain challenges such as those identified above would be useful. Such a heat pump system that can also be used in e.g., a refrigerator appliance would also be useful.